Masks for colour picture tubes are drawn from special sheet metal sheets that are provided with elongated holes. A mask skirting is formed during the drawing and the mask is then made to bulge or arch. Given colour picture tubes with screen diagonals from about 50 cm upwards, the mask skirting is fixed to the frame by means of electric spot welding, using either twelve single spot welds or eight double spot welds as disclosed in German publication DE 3919674A1.
As regards the properties of the jointing between mask and frame, it is desirable for this to be characterized by great rigidity, so that the mask will neither become deformed as a result of impacts nor be excited into vibrations to any substantial extent by sound waves of the kind that will be transmitted by a loudspeaker arranged, for example, inside a television set. On the other hand, however, only a loose attachment is desired, especially in view of the different expansion behaviour of mask and frame during thermal processes employed in the manufacture of colour picture tubes and during the operation of such tubes, especially when substantial beam currents are involved.
With a view to satisfying these conflicting requirements in an optimal manner, it is usual for masks to be provided with stiffening beadings in particular positions, while decoupling slits are arranged elsewhere. Considerable attention must also be paid to the height of the spot weld in relation to the height of the mask. Such general problems and solutions therefor are described, among others, in DE 31 15 799 A1 (See U.S. Pat. No. 4,806,820).
A detailed solution of the problems discussed hereinabove is specified in DE 39 19 674 A1. The proposed solution involves the use of the previously mentioned eight double spot welds in place of the twelve single spot welds. When double spot welds are used, one such double spot weld will be situated in each of the four corners, while the remaining four will be arranged more or less at the mid-points of the four sides. When twelve single spot welds are used, there will be one such weld in each of the positions just described, while the remaining four will be arranged to the right and to the left of the central weld on the two long sides.
Geometric considerations indicate that longitudinal expansion of the mask is particularly critical as far as the proper functioning of a colour picture tube is concerned. Indeed, since the mask is only very slightly arched, changes in the length of the mask will have a considerably enhanced effect on the arching height of the mask. This, in turn, will lead to considerable displacements of the spatial position of the holes in the mask and thus causes shifts of the electron landing spots on the front trough of a colour picture tube. Since the said front trough is provided with a multitude of adjacent and tightly packed phosphor strips with different emission colours, such displacements of the spots struck by the electrons creates a situation in which the electrons no longer strike the particular phosphor strip for which they are really intended. Colours will therefore be falsified.
With a view to keeping these arching changes as small as possible, and therefore also the colour distortions caused by this phenomenon, it has become known to use masks made of invar. Mask-frame combinations with an invar mask are particularly easy to manufacture when the frame is likewise made of invar. However, since invar is several times as costly as steel, the frame of such invar combinations is usually designed as a relatively weak structural member. Notwithstanding this rather undesirable weakening of the frame, pure invar combinations are still considerably more expensive than a steel frame and a mask made of invar. Given combinations of the latter type, it is known for the mask not to be lapped over the frame, which is the usual method of manufacturing such combinations, but to choose the diameters of the mask in such a way as to make it fit into the interior of the frame. The spot welds will then join the mask skirting to the interior surface of the frame. But this calls for rather complicated and therefore failure-prone manufacturing devices, because the mask has to be held from inside and the spot welds have to be applied simultaneously, likewise from inside. By comparison, the devices used in the traditional manufacturing mode are far simpler, because the mask can be supported over a large area from inside, while the spot weld electrodes have free and unencumbered access from outside the frame.
On account of the above description of the problems associated with known mask-frame combinations consisting of a steel frame and an invar mask there existed a need to have these combinations designed in such a manner as to make their manufacture simpler than has hitherto been the case.